High voltage and medium voltage transformers are widely used in electrical power distribution of today. Utilizing the magnetic features of electrical currents, they transfer power between two or more incompatible electrical AC-circuits. Thereby, power from a power plant can be transported by a small current of very high voltage and then stepped down to a large current of low voltage before reaching customers.
Supply authorities are under an obligation to their customers to maintain the supply voltage between certain limits. A tap changer is a device used in a transformer for regulation of the transformer output voltage within these limits. Normally, this is achieved by changing the ratios of the transformers of the system by altering the number of turns in one winding of the appropriate transformer(s). This ratio determines the voltage ratio between the windings and is essential for the stabilization of network voltage under variable load conditions. The tap changer changes the turn ratio between windings in a transformer. An on-load tap changer (OLTC) normally has a regulation range of ±20% of the total line voltage; regulation is performed in roughly 9 to 35 steps and operated 10 to 20 times a day in normal grid applications. For very demanding systems, such as melting furnaces, there may be hundreds of such operations per day.
A lower load on the system may for instance require that tap-changing operations decrease the number of turns in the winding. This ultimately results in an increased output voltage as compared to if no tap changing were performed.
Besides the described application, tap changers may also be used in connection with other inductive power devices such as reactors. Tap changers are either on-load, i.e. operating while the transformer is energized, or off-load and there is a wide range of models available. A tap changer generally comprises a number of switches for tap changing and a number of resistors or other impedances to prevent short-circuiting. Furthermore, the tap changer typically is filled with an insulating liquid, such as oil, which besides insulation offers cooling of the device.
There is also a large demand for tap changers used in industrial transformers in rectifier and furnace applications. In some applications the tap changer may perform several hundred thousand switching operations per year. Phase Shifting Transformers (Management of power flow in AC networks) and Transformers for High Voltage Direct Current (for long distance transmission and coupling of unsynchronized networks) transmission are two other areas where there is an emphasis on voltage regulation.
Power utilities throughout the world are constantly seeking to improve the economic and technical performance of their assets. Needless to say the two go hand in hand and because of the size of the investments required and the long life expectancy of power grid installations there is a healthy skepticism in the industry to new and unproven technology. The emergence of mature vacuum technology is a response to the need for more efficient asset utilization.